The present invention relates to pollution control and, more particularly, to catalytic reduction of nitrogen oxide pollutants in a gas turbine exhaust.
Most internal combustion engines employing hydrocarbon fuels produce power by burning the fuel by reaction with oxygen in the air. As is well known, however, oxygen comprises only about 21 percent by volume of the air. The majority of the remaining 79 percent is nitrogen which does not contribute to the combustion reaction. However, under the conditions in the combustion chamber of an internal combustion engine, the nitrogen tends to react chemically with excess oxygen to produce compounds which are unwelcome pollutants in the exhaust. Such compounds may be NO, NO.sub.2 and higher oxides of nitrogen, all of which are known collectively as NO.sub.x.
NO.sub.x has been identified as a principal intermediate compound in the generation of photochemical smog. When the atmospheric NO.sub.x is irradiated, particularly with ultraviolet light, ozone is released and the characteristic light occlusion, odor and deleterious action ensues.
Because NO.sub.x is such a contributing factor in air pollution, governments have applied increasingly strict standards on its emission from internal combustion engines.
The operating conditions of an internal combustion engine such as, for example, a gas turbine, can be adjusted for minimizing NO.sub.x emissions. However, the adjustment is critically related to the load being driven by the gas turbine engine, and adjustment which minimizes NO.sub.x emissions at one load value is unsatisfactory as the load changes upward and downward. When a gas turbine is employed in an application having a constant output load such as, for example, in driving a generator used as a base load source for an electric utility, reasonable levels of NO.sub.x can be achieved by careful adjustment of operating conditions. The same is not true for a gas turbine employed in a peaking system by an electric utility. By its nature, a peaking system is required to rapidly respond to changes in load both above and below a normal output point. In fact, when a peaking system is operated as spinning reserve, its output load is essentially zero. When an increased demand is sensed by the utility, the peaking system must rapidly respond by increasing its electric power output from zero to some finite value which may then rapidly vary upward and downward with changing load.
The prior art contains disclosure of a number of techniques for reducing atmospheric pollutants in flue gas. For example, U.S. Pat. No. 4,293,521 discloses adding sodium hydroxide (NaOH) to a flue gas for reaction with pollutants to produce solid precipitates which can be removed from the flue gas such as, for example, by a cyclone separator, before the remaining gas is exhausted to atmosphere.
U.S. Pat. No. 3,977,836 discloses the use of ammonia (NH.sub.3) in the presence of a catalyst to reduce NO.sub.x in the flue gas to nitrogen gas plus water. This patent discloses the difficulty of measuring ammonia and, in fact, discusses the measurement of ammonia by reacting it with an excess quantity of NO.sub.x in the presence of a catalyst to determine the amount of ammonia present by the decrease in NO.sub.x.
In a base load system, it would be possible to add NH.sub.3 to the gas turbine exhaust in a molar ratio which would minimize NO.sub.x in the effluent. In order to do so, measurement of NO.sub.x in the effluent would be used as a guide in adjusting the flow of NH.sub.3 into the gas turbine exhaust. However, measurement of NO.sub.x with available analyzers such as, for example, chemi-luminescent infrared or constant potential electrolytical techniques are relatively slow, requiring on the order of a minute or more for completion not including the transport time of the gas from the sensing location to an analyzer. Under a rapidly changing load, response times on this order may permit the discharge of excessive NO.sub.x or NH.sub.3.
Residual ammonia in the effluent of a gas turbine represents a significant pollution factor on its own.
NO.sub.x emission standards are being applied in some locations in the world which exceed the ability of even an ammonia and catalyst system operating as previously described to achieve.
The thermal efficiency of a system employing a gas turbine can be significantly improved by recovering the waste heat in the gas turbine exhaust for the production of steam and by using this steam to run a steam turbine. Some steam turbine and gas turbine combined cycle systems known under the General Electric Co. trademark STAG employ a heat recovery steam generator (HRSG) through which the hot gas turbine exhaust passes on its way to the atmosphere. One or more stages of economizer and steam generator as well as possible superheaters are employed in the heat recovery steam generator for feeding a steam turbine of one or more stages. The outputs of the steam and gas turbines may be combined in a single load or, alternatively, may be applied to different loads. One may be employed to drive an electric generator, and the other employed to power other apparatus. Alternatively, both turbines may be coupled to the rotor of the same electric generator for combining the power output. Other applications include the generation of electricity by the gas turbine and the use of the steam for non-motive power such as for heating or industrial processes.
As the exhaust gas from the gas turbine passes through the heat recovery steam generator, its temperature is reduced from the range of from about 800.degree. F. to about 1,100.degree. F. to about 300.degree. F. by heat transfer to the steam generators and economizers. The catalytic reactor is located in the HRSG and is designed to operate efficiently within the aforesaid temperature range.
Automatic control systems for gas turbines make available a number of measured and calculated operating parameters. U.S. Pat. No. 3,520,133, herein incorporated by reference, discloses one type of automatic gas turbine control system.